Psychiatrically disabled homeless adults (PDHAs) lead complex, troubled lives. Although much has been written about their needs, research from the service user's perspective remains virtually unknown. Our goal is to develop a social ecology model that addresses poorly understood 'process' questions posed by the juxtaposition of two distinct approaches: 'treatment first' vs. 'housing first'. What factors--person-based and contextual--impede or enhance engagement and retention in care among PDHAs enrolled in these two program models? We propose to use qualitative methods and an interdisciplinary research team to investigate the service delivery system from the 'native's point of view'. Specific aims are: 1) To map 'successful' vs. 'unsuccessful' life course trajectories of engagement and retention in care among PDHAs previously enrolled in two contrasting programs using case study methods (N--40); 2) To conduct a longitudinal study of PDHAs enrolled in 'housing first' vs. 'treatment first' programs to learn how engagement and retention in care vary when program philosophies and practices fundamentally differ. This aim will be accomplished in two ways: a) a prospective study in which 80 enrollees in three programs serving PDHAs in New York City are followed for one year; b) in-depth interviews with service providers for PDHAs to identify areas of congruence/non-congruence with consumer self-reports and perspectives. Providers include: R's case managers (n=80), case manager focus groups at the 3 program sites (n=30), and experienced professionals serving PDHAs (n=10); 3) To specify viable changes in delivery of services that would result in successful engagement and retention of PDHAs in care for mental illness and co-morbid substance abuse. Expert panels will be convened in which findings from this study are discussed to develop suggestions for program/policy improvements and identify barriers to implementation. Our ultimate goal is to translate empirical findings into replicable policies and practices that will enhance outreach and service delivery for this 'hard-to-reach' population.